I think Starbucks works for the same reason McDonalds works. Their logo is a seal of okayness that works across the globe.

No matter where I am driving in Europe, I know I can step off the highway and find a McDonalds with air conditioning, food, ice cream and Wi-Fi. I know I can use their touch screen terminals and pay with my card, regardless of the local language or currency. It's a meal for when you don't want to think about having a meal. It's logistically convenient.

I suspect Starbucks is the same.